As the bacterium Bacillus subtilis differentiates from the vegetative form into a dormant endospore, complex morphological and physiological changes occur which require the sequential expression of many genes. During the process, new RNA polymerase sigma subunits appear, which displace one another in a sequential cascade, confering on the RNA polymerase a changing specificity for the recognition of different classes of promoters. This mechanism of altering transcriptional specificity may be a fundamental element in the regulation of sporulation gene expression. Experiments are proposed to determine whether the sporulation specific sigma subunits are actually necessary for spore formation and we will attempt to identify non-sigma regulatory factors which may be required in addition. I have defined the promoters for two genes which are transcribed by a novel form of RNA polymerase containing a 37,000 d sigma. Mutagenesis of these promoters together with methylation protection and chemical crosslinking experiments will be used to test the hypothesis that the sigma subunit directly contacts nucleotides at the -10 region and the -35 region of a promoter enabling the RNA polymerase to bind to the promoter.